A high current is required for certain applications. Energy conservation has increased the demand for Light-Emitting Diode (LED) lighting to replace traditional lamps. A very high current such as half of an amp may need to be driven through one or more LED's to provide the desired luminosity. The power source may be an Alternating-Current (A.C.) source such as provided by an electric utility or portable generator or alternator on a moving vehicle.
A rectifier such as a full diode bridge and a transformer may be used to convert power and drive the LED or other load. A residual of the A.C. input may appear on the output as a variation or ripple that may match the A.C. frequency, such as 120 or 100 Hz. This ripple may be reduced by using a capacitor across the diode bridge. However, when large currents are used to drive loads such as LED's, this capacitor must be very large. However, having a large capacitor on the primary side may reduce the power factor, since the primary-side rectified AC waveform is distorted and not similar to the input voltage to the diode bridge.
An electrolytic capacitor is often used for reducing ripple. Since the primary side of the transformer typically has a high voltage, a high-voltage electrolytic capacitor is needed. However, these high-voltage electrolytic capacitors are prone to failure. A short or leakage in the high-voltage electrolytic capacitor used to reduce ripple may cause the LED driver circuit to fail, reducing the rated lifetime of the LED lighting product. Although the LED itself has a high lifetime, the high-voltage electrolytic capacitor on the primary side of the LED driver circuit may reduce the lifetime of the overall LED lighting product. Thus removing the high-voltage electrolytic capacitor is desired. Reducing ripple using a circuit that uses smaller, low-voltage capacitors is desirable. Thus if the driver circuit can be modified to reduce output ripple significantly, without a high voltage electrolytic capacitor on primary side, the lifetime of the LED lighting product will be increased.
FIG. 1 is a schematic diagram of a prior-art converter with a high-voltage primary-side capacitor. AC supply 15 produces an alternating-current that is applied to a full-wave rectifier bridge of diodes 11. The output of the bridge of diodes 11 is connected to input voltage VIN and ground. Primary capacitor 17 acts to store current and smooth out variations such as ripple in VIN. For example, a 240-volt AC supply can produce a peak 370-volt signal for VIN. AC supply 15 could be a wall electrical output that is connected to a domestic AC supply or to an AC generator.
Transformer 10 can have an iron core to enhanced mutual inductance between primary windings that are connected between input voltage VIN and ground, and secondary windings connected to secondary diode 26.
The primary loop of transformer 10 has primary current flowing from primary capacitor 17 to VIN, then through the primary windings of transformer 10 to ground. A resistor or switched transistor such as a MOSFET (not shown) may also be present in this primary loop.
The secondary loop of transformer 10 has secondary current IS flowing from the secondary windings of transformer 10 through forward-biased secondary diode 26 to output voltage VO. Secondary capacitor 28 stores charge to provide a more constant current through load resistor 25 when secondary current IS is not flowing from transformer 10.
Primary capacitor 17 is usually bulky in size for high-current drivers such as LED drivers. For example, primary capacitor 17 may have a capacitance value of 100 uF (450V). High voltage electrolytic capacitors are bulky compared with low voltage capacitors. Traditionally, the size of a 22 uF high voltage electrolytic capacitor is about the same size as a 35V, 470 uF capacitor. Typically, primary capacitor 17 is a 22 uF capacitance for a 10 W LED driver. These values require large bulky capacitors such as an electrolytic capacitor. Also, since the primary side has a high voltage, such as 240 volts, a high-voltage capacitor is needed to withstand the high voltage across its dielectric. These high-voltage electrolytic capacitors are not only expensive, but also failure prone. Leakage in the high-voltage electrolytic capacitors can limit the rated lifetime of the overall LED circuit.
Opto-couplers are sometimes used in LED driver circuits. However, the opto-coupler may degrade over time, such as by dirt accumulation that blocks light. Thus a LED driver circuit that does not use an opto-isolator is also desired.
Control circuitry is needed to produce a constant DC output current. A transformer may be used to isolate the primary side connected to the AC power from the DC output side for enhanced safety. Rather than connect the control circuitry to the secondary (DC) side of the transformer, the control circuitry can connect to the primary (AC) side. Heat dissipation, form factors, component counts, and costs can be reduced with primary-side sensing regulators (PSR) and control due to the higher efficiency and elimination of the opto-isolator.
FIG. 2 shows a converter without a high-voltage primary capacitor. A power converter that reduces ripple while reducing or eliminating the primary-side high-voltage electrolytic capacitor is desirable.